1. Field of the Invention
The present invention relates in a copying machine of the type which comprises one or more reflective metal surfaces each having a multilayer film to compensate for the mismatch in spectral sensitivity characteristics between the light source and photosensitive medium in the copying machine.
2. Description of the Prior Art
In the art of copying machines and the like it is well known that an original in a certain color can not be copied due to the particular wavelength characteristics which the light source shows and the particular sensitivity characteristics which the photosensitive medium shows in the optical system of the copying machine. Here and hereinafter the term "optical system" should be understood to include light source, photosensitive medium and other optical members commonly used in a copying machine.
This unfavorable phenomenon is seen, for example, when a halogen lamp is used as the light source and a CdS series photosensitive medium is used as the light receiving part. In this case, characters and figures written in red on the original come out very faintly or do not come out at all in the copy as far as the red part is concerned. This is attributable to a combined effect of the wavelength characteristics of the light source and the sensitivity characteristics of the photosensitive medium. A halogen lamp has its maximum emission energy in the infrared spectral range ranging from 800 m.mu. to 900 m.mu. at the filament temperature of about 3000.degree. K normally used. The emission energy decreases gradually at a constant rate toward the short wavelength side. On the other hand, CdS photosensitive medium has such spectral sensitivity which becomes high in the range of from red to near infrared. As a result, characters and figures in red on an original are overexposed to light as compared with those in blue and green on the same original.
A similar phenomenon is seen also in the optical system comprising a fluorescent lamp as the light source and a Se photosensitive medium as the light receiving part. In this case, since the fluorescent lamp is rich in blue energy and the sensitivity of Se photosensitive medium is particularly high to blue, there is caused an overexposure to blue light so that characters and figures written in blue in the original are difficult to copy.
Therefore, some means should be provided to reduce or eliminate the above mentioned phenomenon whenever there exists a mismatch between the wavelength characteristics of light beam entering the optical system and the sensitivity characteristics of the photosensitive medium then used.
One of the known solutions to the problem is to interpose an absorption filter having wavelength selectivity to light in the optical path. Because of expensiveness, such selective absorption filter is generally disposed at a position where the beam diameter becomes minimum in the optical path extending from the light source to the photosensitive medium. For example, it is positioned in or near the imaging lens. DOS No. 2,350,281 (filed on Oct. 6, 1973) has disclosed a reflection type projection lens system having a built-in filter. However, as pointed out in the specification by the inventor himself, the novel lens system disclosed therein has a disadvantage that the transmissivity of the light beam passing through the lens system is greatly decreased. This is because the imaging beam of light has to pass through the filter twice, once each at entrance time and at exit time. Another disadvantage of the solution is that the image formation power of the lens system is substantially reduced by reflection of light on the filter surface.
It is also known to interpose a dielectric multilayer film as a reflection mirror or filter in the optical path so as to adjust the quantity of light for every wavelength. One of such multilayer films is a dielectric multilayer interference film which possesses a property similar to that of a dichroic mirror for color separation used in a color television camera. However, this reflective multilayer film is low in reflectance or reflection factor. In order to obtain the same degree of reflection factor as that of a simple metal reflection mirror, the reflective multilayer film must be composed of ten or more dielectric layers which makes the film expensive.
We, the applicants of the present application have already proposed in our prior application, U.S. application Ser. No. 964,986 (filed on Nov. 30, 1978) that the sensitivity characteristics of the photosensitive medium should be compensated by a novel type of multilayer interference film. The film is prepared by laminating alternate high and low refractive layers on a glass substrate, each refractive layer having an optical film thickness corresponding to 1/4 of the design wavelength. Generally, glass is a material of low reflectance. The prior invention aimed at increasing the reflectance excepting a predetermined range of wavelength by providing such multilayer interference film on a glass substrate. The present invention aims at attaining the same effect in another way. According to the invention there is used, as the substrate, not glass but a metal surface which has a higher reflectance than glass. By using a high reflective metal surface as the substrate and reducing the reflectance selectively for a predetermined range of wavelength, the same effect aimed at by the prior invention can be attained. The metal surface used as a substrate in the invention is formed, for example, by vapour depositing a metal film on a glass plate. In other words, the metal surface is generally supported by glass.